Making a hole in a Dynatopo sculpt without Boolean
You can try to use a more advanced boolean solver. there's a plugin somewhere that uses the CarveMT solver to make more complex cuts

The biggest villain in boolean operations is that it comes to a point blender doesn't know how to deal with all the vertices and all the new geometry it has to create to accomodate the change, be it a union or a cut. If possible, decimating or reducing the mesh density in the area of the cut will increase the chances of success